// 前端Ajax請求示例
$('#username').on('blur', function() {
  $.ajax({
    url: '/api/check_username',
    type: 'POST',
    data: {username: $(this).val()},
    success: function(res) {
      if(res.available) {
        showSuccess('用戶名可用！');
      } else {
        showError('該用戶名已被注冊');
      }
    }
  });
});

// 后端處理邏輯（Node.js示例）
app.post('/api/check_username', async (req, res) => {
  const {username} = req.body;
  const exists = await db.query('SELECT * FROM users WHERE username = ?', [username]);
  res.json({available: !exists.length});
});